rf design considerations for 802.15.4 hardware … · proper antenna orientation is required to...

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RF Design Considerations for 802.15.4 Hardware Development

June, 2010

Mark WilliamsApplications Manager

FTF-ENT-F0515


Course Objectives

►After completion of this course you should:• Understand the requirements of RF and wireless design• Understand the function of antennas and RF matching networks• Understand the special requirements of RF PCB layout• Understand the benefits of using Freescale’s 802.15.4

reference designs• Understand how to verify your design• Understand the need for, and requirements of, certification

►This course will NOT make you an RF engineer. But you will gain an appreciation for the specialty of RF engineering, and be able to provide guidance for experienced hardware engineers to create successful 802.15.4 designs

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Agenda

► Introduction

►RF Hardware Design Philosophy

►Antennas and RF Matching Networks

►Design and Board Layout Considerations

►The Use of Reference Designs

►Design Verification Methodology

►Certification

►Summary

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Introduction

►Why does RF design seem like “black magic”?• Parasitics that can be ignored in low-frequency analog and digital circuits

have a major impact on radio frequency circuit performance

• Models are usually much less accurate, requiring more empirical design work

• Wireless hardware designs (cellular, Bluetooth, Wi-Fi, 802.15.4, etc.) are “intentional radiators” so they have special certification requirements

• Design and test of wireless hardware requires special equipment and skill for digital and analog designs

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RF Hardware Design Philosophy

►Every trace on a circuit board is a circuit element that cannot be ignored• Circuit traces become transmission lines of a given electrical length• Short lengths of line tend to be capacitive, longer length inductive

►Ground connections are difficult to create since “every trace is a circuit element”

►Any conductor will be a radiator — i.e. an antenna — at RF frequencies

►RF design techniques need to be applied above a few hundred MHz

►Chip components become complex circuits above a few hundred MHz

►High power RF (> 20 dBm) accentuates these issues

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Antennas and RF Matching

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What Is an Antenna?

► IEEE®: A means for radiating or receiving radio waves.

►Sayre: To convert RF energy (alternating current) into electromagnetic waves (or vice-versa), as efficiently as possible.

►Skolnik: A transducer between free space propagation and guided wave propagation.

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What is an Antenna?

► Antennas are used at frequencies as low as 30 Hz (submarine ELF, extremely low frequency) up into the hundreds of GHz (EHF, extremely high frequency). This presentation focuses on 2.4 GHz applications.

► The characteristics of the antenna (gain, impedance, polarization, size, cost, etc.) should suit the application.

► This means that an antenna suitable for one application may not be efficient for another.

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Antenna Selection Considerations

►When selecting an antenna, one must consider:

• Cost • Size • Gain/directivity (pattern)• Reliability (durability)• Manufacturability• Bandwidth• Polarization• Input impedance• Power handling • Connector type (if any)

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Antenna Selection Considerations

► In a 802.15.4 applications, cost, size and pattern are usually the most important.

►A few designs, such as the inverted F-antenna or bent dipole, will be suitable for most ZigBee® applications that use a PCB antenna.

►Special designs may sometimes be needed, such as a chip antennas due to size limitations, or an external whip for outdoor applications.

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Antenna Basics

►Antenna Gain • dBi is the most common reference used

for antenna gain. This is gain referenced to an “isotropic” antenna, which is a theoretical perfect sphere which radiates equally in all directions (0 dB gain in all directions)

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Antenna Basics

►No such antenna actually exists, but it serves as a good reference standard.

►Sometimes dBd, which is gain referenced to a horizontal dipole in space, is used. Most references define 1 dBd = 2.14 dBi. A horizontal dipole at the correct spacing above a ground plane can have gain greater than 2.14 dBi.

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Antenna Terms

► Peak gain: The maximum gain of an antenna.

► Average gain: The average of antenna in all directions.

► Efficiency: The ratio of radiated power to supplied power. Typically 20% to 90%.

► Null: A significant dip in the gain of the antenna in a particular direction.

► Directivity: The power in the ”main beam” or highest gain direction of an antenna compared to isotropic, expressed as a ratio.

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Is High Antenna Gain Always Good?

►Gain in one direction is only obtained by reducing gain in another direction. That is, a “high gain” antenna only has gain in one or more directions. In other directions, the gain is significantly reduced (nulls).

►Proper antenna orientation is required to ensure that the gain peak is aimed towards the desired signal.

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Is High Antenna Gain Always Good?

►So, high gain is not always good(!). For handheld or portable applications where the orientation of the device is not controlled, a high-gain, highly directional antenna may not work very well for the end user.

►High-gain antennas can work very well in fixed applications.

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(from TP8347)


The Dipole

►The horizontal ½ wave dipole is the most basic antenna.

►The radiation pattern is ”doughnut” shaped, a null from the end of each radiator. The impedance at the feedpoint is 73 ohms for a perfect theoretical dipole, at the resonant frequency.

►The dipole is a balanced, differential structure.

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The basic dipole:

Total length is ½ wavelength,(At 2.4 GHz, approx.6.1 cm in air)


Bent Dipole

►The ends of a dipole can be bent to reduce the horizontal space required.

►By bending the dipole, the end nulls will often be filled in, at the expense of some forward gain.

►Changing the shape may also change the impedance and/or the length at resonance.

►The Freescale 13192 SARD used a bent dipole antenna.

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Dipole shapes to reduce width and improve the omnidirectional characteristics:


The Monopole

► By substituting one leg of the dipole with a conductive ground plane, the remaining part of the dipole ”mirrors” itself in the ground. For a perfect infinite ground plane, the feedpoint impedance is half that of a horizontal dipole, or 36 ohms.

► The ground plane can be the ground plane of a PCB board or metal case.

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A monopole above a groundplane, showing the “mirror”antenna.


The Monopole is VERY Ground Plane Dependent

►A ground plane needs to be approx. 6 times the wavelength in diameter to act as a theoretically ”infinite” ground plane.

►Anything smaller will change the pattern from the ideal. In general, as the ground plane gets smaller, the peak in the pattern moves up from horizontal, to as high as 30 - 40 degrees above horizontal.

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Inverted F-Antenna

►By bending the monopole into an inverted L shape, the omnidirectional characteristics are improved, but efficiency is slightly lower and impedance is much lower.

►By ”tapping” the monopole we get the inverted F-antenna with good omnidirectional characteristics and a 50 ohm impedance point.

►The F-antenna is a good all-around antenna. It is very commonly used, including on Freescale 802.15.4 evaluation boards.

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Tilted whip and F-antenna:(note the ground plane area)


Other Antenna Types

►Many other antenna types exist. Patch antennas have a patch of metal over a ground plane, and are very directional (used for GPS). Loop antennas are also used in PCB applications. Slot antennas can be thought of as dipoles cut out in metal plates. They are often used in microwave applications.

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Half-wave and full-wave loops:

Half-wave and quarter-wave slot antennas: Patch antennas:A metal patch over a ground plane


Chip Antennas

►A lot of different chip antennas exist on the market. The usual principles of operation are monopoles, helixes and patch antennas.

►The antennas are made of a low-loss ceramic with high dielectric constant. The high dielectric constant (approx. er=10) shortens the physical length of a wavelength in the material.

►Be careful when comparing size: The actual chip antenna may be very small, but often requires a much larger cleared area and ground plane to be efficient.

►Typical gain ranges from –10 to –2 dBi. Good for very short range applications, such as wireless mouse, keyboard, some remote controls, etc.

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(Johanson Technology)


Printed Antenna Use Considerations

►Antenna design requires specialized tools and experience• Don’t change a design unless you know what you are doing and are

ready for multiple board spins and time on the bench►A design is optimized for a particular PCB material and thickness.

Changing either will impact the design►Metal thickness is usually not a concern►Grounding is critical and ground vias are part of the antenna design►Follow a reference design EXACTLY

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Using Chip Antennas

► Just placing a chip antenna on a PCB layout does not ensure good performance. The bandwidth of the chip itself is very narrow. External matching circuits are used to broaden the response (and center it in-band). Also consider:• Each design must be tuned on the layout and case for which it is placed.• Performance can vary due to production variations.• Consult closely with the manufacturer for applications guidance.

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Johanson Technology, Chip Antenna Layout Considerations for 802.11 ApplicationCenturion Technologies, NanoAntTM Application NoteMurata Manufacturing Company, Application Note of Chip Dielectric Antenna M1 Series/W1 SeriesMitsubishi Materials Corp., TRJC-2F03, Recommended Mount Conditions for 2.4 GHzPulse/Technitrol, Application Note, WLAN 2.4 GHz. AntennaYageo, High Frequency Products (lf28429.pdf)


“Antenna” Matching

►What is normally referred to as “antenna” matching is actually matching performed for the benefit of the transmitter.

►The impedance of the antenna is transformed to the TX output impedance, giving maximum power transfer and best overall performance.

►The matching does not usually improve the performance of the antenna — it delivers more TX power into the antenna.

• Return loss: A measure of impedance match. Essentially, the loss in power reflected from the load. Can also be expressed as VSWR.

• Bandwidth: The frequency range over which the return loss is acceptable.• Input impedance: The RF resistance of the antenna at resonance (Fo). The impedance will

vary at other frequencies.• Resonance: Frequency where the antenna impedance is purely resistive

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Matching vs. Tuning

► To tune an antenna, the physical dimensions of the antenna are optimized so that the antenna itself is resonant as intended.

► Antenna tune quality is normally measured by measuring reflected power vs. forward power, or return loss, at the desired frequency(s) of operation. If an antenna is very lossy, it may not work very well but indicate a good return loss. But, in most cases, return loss can be used to determine the center frequency of an antenna.

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Matching vs. Tuning

► Additional matching may be required if the impedance of the antenna, when tuned, is not the same as the TX output impedance — for example, to match a 36 ohm antenna to a 50 ohm RF circuit. Matching is normally accomplished using passive capacitors and inductors that are intended for use at 2.4 GHz.

► In some cases, it may be easier to match and tune at the same time. For example, an antenna that is too short can be matched to 50 ohms in the desired band with series inductance at the input (such as citizens band radio antenna with a large coil at the base).

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Balanced-Unbalanced Circuits

► In many cases, the RF transceiver will have an interface that is balanced — it has a positive and negative output. This architecture has advantages at the die level, but usually must be converted to single-ended (one side referenced to ground) for most RF circuits. Some sort of conversion circuit is necessary.

► This can be done with a commercial ceramic balun, as shown below (Z100).It can also be performed with a lumped element balun (L/C).

► If the antenna is balanced, such as a dipole, and there are no other single-ended parts such as a TX/RX switch, PA, etc., then it would be possible to stay balanced thoughout and not use a balun (such as the Freescale SARD board).

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Antennas: Additional Thoughts

►Steps to good antenna performance• The antenna should be reasonably clear of metallic objects, but oriented

properly with the ground plane (when one is utilized).• Always check the antenna in its final environment, including the PCB,

components, case, hand effects (if appropriate), battery, etc. Plastic and other materials in the near-field may cause detuning.

• Test actual antenna performance by whatever means available, such as range testing, measuring radiated signal level under controlled conditions, and/or send it out for testing in an anechoic chamber.

• Test samples from volume production to check for production variations.

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Antennas: Additional Thoughts

►Steps to good antenna performance• The plastic case will detune the antenna, if the plastic is close enough:

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PCB inverted F, no case PCB inverted F, plastic case in place


Design and Board Layout Considerations

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Assess Your RF and Wireless Capability

►Successful wireless hardware development requires some level of RF skill.• RF layout• Antenna design• RF measurement capability

►You wouldn’t hire an analog designer to write embedded C code, so don’t expect digital hardware engineers to be RF experts.

►Decide whether you want to develop RF hardware expertise and, if so, to what level.

• With investment in some RF test equipment, most hardware engineers can make basic RF measurements

• RF circuit design, layout and antenna design are specialties requiring investment in tools and experience

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Successful Design Approach

►Select a chipset vendor with a proven track record• Basic modem performance is a function of the IC design, and an IC

designed for a particular protocol will generally meet the requirements • Range, output power, harmonic and spurious radiation can all be

affected by the design implementation

►Don’t attempt a discrete or low-level integration design unless you know what you are doing.

►Use the manufacturer-recommended reference designs• These have typically been certified and have optimized performance• Follow the design EXACTLY in the RF region of the board

This does not mean the schematic only, but the LAYOUT• Mixed mode regions are ok to modify, as they generally do not

contribute to RF performance

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Antenna and Connectors

►Regulatory agencies frown on detachable antennas

►Only “unique” connector types and designated antennas will pass certification

► Integrated antennas or those that are not removable are the easiest to certify

► Issues:• Some testing is conducted, i.e. with

coax cable• Radiated testing is difficult• Ultra-miniature coax connectors with

in-line switches are usually considered “unique,” allowing easy testing, but they can impact RF performance.

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Modules as an Alternative

►FCC and other regulatory agencies allow “modular” certification: OEM can use a certified module and does not have to re-certify.

• EXCEPTION: If the radio transmitters are within 20 cm of each other, the hardware must be re-certified.

►Excellent alternative to OEM design for low-volume applications or where RF hardware expertise is lacking.

►Most chipset manufacturers have module partners.

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Digital vs. Analog vs. RF PCB Considerations

►Each discipline has its own considerations and priorities in PCB layout• Digital

RoutingBalanceSignal isolationGrounding

• AnalogBalance Shielding Trance loss Grounding

• RFParasitics Controlled impedance Coupling Grounding

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RF Layout Considerations

►Non-controlled impedance traces should be kept short• Length traces are lossy and inductive

►Grounding is imperative• Remember the antenna considerations?• Lengths of line are NOT ground but inductors• Consider the concept of “earth” or “chassis” ground

►PC traces are effective radiators at RF frequencies because they are a non-negligible fraction of a wavelength

►Examples of PCB Parasitics:• At 2.4 GHz, a 10 mil wide PCB trace 275 mils long on 32 mil FR4 is equivalent

to a 3.2 nH inductor, +j73 Ohms.• A 10 mil via in 32 mil FR4 is about 0.5 to 1 nH.

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Critical Layout Areas

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Critical RF Circuit AreaCopy Exactly

Less criticalCan be modified


Critical Grounding and Layout

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Via to formground planefor antenna

Ground viaunder and near the IC

Controlled impedance line for long runs

Crystal close to the IC


Mechanical Considerations

►Consult Freescale’s ZHDCRM, ZigBee Hardware Design Considerations Reference Manual

• Includes details of recommended footprints, device marking and soldering information for 802.15.4 platforms

• Some of our reference designs and development hardware may deviate slightly from these due to CAD issues or contract manufacturer changes. Either are acceptable

►Soldering of QFN and LGA packages is challenging• Work with a reputable house with experience• Follow their guidelines for solder stencil design• Filled vias are recommended for small vias-in-pads such as with MC13224V• Use a minimum of 5 mils solder thickness to avoid “starvation” of

leaded packages

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Use of Reference Designs

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What are Reference Designs?

►A form-factor design example, essentially ready to build

►Proven performance

►Excellent starting point for OEM design

►Differ from development boards in that the circuitry is simpler and functionally oriented

►Freescale has reference designs for all platforms

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3.8”

3.0”

Interface – UART, I2C, KBI


► MC1321x SRB Development Board: Designed for lab use, code development and experimentation. Lots of stuff….

► MC1321x IPB: Basic RF layout with critical components. Interfaces are pinned out. Designed to be a starting point for OEM designs. Also for add-on to existing hardware

Development Boards vs. Reference Designs

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►Available from the web: www.freescale.com/802154under development tools

Freescale 802.15.4 Reference Design Download

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http://www.freescale.com/802154


Available from the Download Page…

►Numbering Scheme: Platform – Interface, Antenna, Implementation• Interfaces: I = I2C, UART, U = USB • Antennas: P = Printer, C = Chip• Exception: ERB = Extended Range Board

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The Download Contains…

►ZIP file with:• Design files

CAD files from OrCAD or AllegroGerbersPDFs of schematic and layers

• DocumentationReference manualBOM

• SoftwareBeeKit configuration file

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Successful Use of Reference Designs

►Copy the RF and Analog area EXACTLY

• This is the part of the design most likely to fail

• Use the same PCB material and thickness

• Changing the PCB thickness WILL affect the antenna requiring tuning

• Copy the IC footprint

►Digital circuitry can be adapted to the target application

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Critical areaCopy exactly

Less criticalCan be modified


Design Verification Methodology

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When You Get Your First Prototype Back…

►Perform a basic “smoke” test with a current-limited power supply• Does the unit turn on?• Does it draw the expected current?

►Check the RF performance• Use Freescale’s SMAC software or Test Tool (or your own code) • Check the reference oscillator frequency using a frequency counter

CLKO pin on MC1321x family, TMR2 and connectivity test on MC13224VShould be within 10 ppm of the target frequency

• Measure output power using a power meter or spectrum analyzer May require board modification to make conducted measurementsExpect 1-2 dB variation from data sheet specs and from unit to unit

• Measure the range between two boards using packet error rate (PER) softwareOpen space (outdoors) range for Freescale’s reference designs should be 200-300 meters

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Testing and Verification Help

►Consider using a hardware development partner if you do not have RF test expertise

Or…

►Work with a test equipment company such as Agilent or National Instruments

►Plan to spend time in the lab and money on test equipment

►Check out new Freescale RF Test Reference Manual: • ZRFETRM, Freescale IEEE 802.15.4/ZigBee Node RF Evaluation

and Test Reference Manual• Includes details on how to evaluate your board• Will be updated to include a section on production testing

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Moving on to Pre- Certification Testing

►Once your design is working, there are some basic measurements that should be made prior to certification testing

►You may want to work with your certification lab on your first hardware design and have them show you how to make the measurements

►FCC Publication 558074 gives more details on measuring digital radios

►Tests are performed in continuous modulation transmission mode (not packet) or PRBS9 random data packet mode

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Certification

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What is Certification?

► International Telecommunication Union, a branch of the United Nations, provides guidelines for RF spectrum allocations

►Sovereign nations have the right to control radio transmissions within their borders

►Certification is the process of testing radio hardware to demonstrate that it meets the stated regulations in the country that it will operate in

►Within this session, we will address intentional radiators, or RF transmitters, operating in the 2400 MHz “unlicensed” bands

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When is Certification Required?

►Generally, when electronic hardware will be sold in a country, the certification requirements of that country must be met.

►Certification is usually only required once unless changes are made to the hardware that will affect the RF emission performance.

• Examples: IC changes and software revisions that do not change radio performance are acceptable. (See CFR 47 FCC Part 2.1043)

►Some countries such as Japan require filing notification with the regulatory agency even if engineering or demonstrations will be performed with RF-emitting hardware.

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What Regulations Apply?

►For operation in the 2.4 GHz band (worldwide):• In the U.S., CFR 47 FCC Part 15.203, 15.205, 15.209 and 15.247• In Canada, RSS-210 which closely follows Part 15• In EU, ETSI EN 300, 301• In Japan, ARIB STD-T66• Other countries generally follow FCC or ETSI

►Consult an expert!• Hire a respected test and certification lab that has experience with unlicensed

band hardware and can file (or help you file) in the countries of interest• Hardware labeling and documentation are critical

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Pre-Certification Testing

►Following are some recommended pre-certification tests that should be performed as part of the design verification.

►FCC Publication 558074 gives more details on measuring digital radios

►Tests are performed using Freescale’s SMAC test mode or connectivity applications in continuous modulation transmission mode (not packet) or PRBS9 random data packet mode

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Peak Output Power

►Maximum output power at which your unit will operate

►FCC Part 15.247 (b) (3) applies with a limit of 30 dBm (measured conducted) and a 6 dBi (dB relative to an isotropic radiator) gain antenna

►Test conditions• 10 MHz RBW and VBW• Peak detector• Max hold• NOT a band power measurement• Low, mid and high channel• Continuous modulated transmit

mode (not packet)

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Spurious Emissions - Conducted

► Includes harmonics, spurs and spectral mask

►Part 15.247 (c) applies with limit of -20 dBc

►Test conditions:• 100 kHz RBW, 300 kHz VBW• Peak detector• Auto sweep with max hold• Full sweep and band edge• Continuous modulation mode

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Spurious Emissions - Radiated

►Similar to spurious conducted; however, measured in a 3 meter RF chamber

►Part 15.247 (c) along with limits in 15.209 and 15.205: 500 µV/m (54 dBµV/m) for average detector since “forbidden bands” are on either side of 2.4 GHz (2.2 – 2.3, 2.31 – 2.39, 2.483 – 2.5 GHz)

►Most OEM labs cannot make radiated measurements, soconducted measurements with antenna gain can be substituted

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Frequency range of Harmonic or SpuriousEmission

Average Limit Converted from Fieldstrength to dBm measured at RF port

Average Fieldstrength Limit of Fundamental (dBμV/m @ 3m)

Peak Field Strength Limit of Fundamental (dBμV/m @ 3m)

Peak Limit Converted from Field strength to dBm measured at RF port

Above 960 MHz -41.12 54 74 -21.2


Spurious Emissions – Radiated (con’t)

►FCC Publication 558074 specifies continuous modulation mode, not packet

► If the average limit is applied, a duty cycle correction factor can be used per 15.35 (b): On time divided by off time in 100 ms

►Since 802.15.4 is a burst-mode, low duty cycle protocol, a reasonable compromise between ~75 kbps throughput and ZigBee Alliance specification of no more than 10% duty cycle is 17%. One full packet transmitted, no acknowledgement received, the packet is retransmitted 3 more times with no acknowledgement:

• Maximum 15.4 packet contains 133 bytes• At 250 kbps data rate, Tb = 4 µs and TB = 32 µs• 133 byte packet lasts 4.26 ms• 4 transmitted packets is total on time of 17 ms

►Alternatively, peak limits and conditions can be used. The peak limit can be applied with no correction factor.

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Spurious Emissions – Radiated (con’t)

►Average measurement (typically what most labs do):

• 1 MHz RBW• 10 Hz VBW• Auto sweep with max hold• Continuous modulated• Duty cycle correction can

be applied

►Peak measurement:• 1 MHz RBW• 3 MHz VBW• Auto sweep with max hold• Non continuous mode

►DO NOT use dBµV mode on analyzer – does not correct for radiated power

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Duty Cycle Correction Factor

►To use a duty cycle correction factor, the duty cycle must be demonstrated

►Use PBRS9 transmit mode►With spectrum analyzer in zero span, measure on time

and off time in 100 ms

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Power Spectral Density

►Not normally an issue as this is set by the modem and transmitter

►FCC Part 15.247 (d) applies with a limit of 8 dBm/3 kHz

►Must use PRBS9 transmit mode!

►Test conditions• 3 kHz RBW• VBW > RBW• Sweep = 500 seconds• Max hold• Span = 1.5 MHz

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Summary of Test Modes

►Continuous transmit modes are available in Freescale SMAC test software (test mode app and connectivity test)

►For duty cycle correction factor measurement, use PRBS9 mode

►For power spectral density measurement, use PRBS9 mode

►For spurious measurements, use continuous modulated mode

►For peak power measurements, use continuous modulated mode

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Suggested Reading

►CFR 47 FCC Part 15 – available from the FCC website• Sections 35, 205, 209, 247

►CFR 47 FCC Part 2.1043 – available from the FCC website• Allowed changes without recertification

►FCC Publication 558074 – available from the FCC website (use Google search)

►“Simplifying FCC Compliance for 802.15.4 2.4 GHz Devices” –White paper by Glen Moore, Wireless/EMC Manager, National Technical Systems

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Summary

►Work with an experienced certification lab to help you determine which certification requirements apply to your product

►Realistically assess your RF capability and goals

►Select a proven chipset vendor and follow their reference designs exactly for RF layout

►Perform pre-certification testing or have your certification partner perform the tests to insure you pass before finalizing your design and formally submitting it

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Summary

► In this session we have reviewed:• RF hardware design philosophy

Details of how RF/wireless hardware design differs from digital and analog• Antennas and RF matching networks

Basic information on terminology, design considerations and antenna options• Design and board layout considerations

Guidelines and references• The use of reference designs

FSL 802.15.4 reference designs as a valuable OEM design starting point• Design verification methodology

The steps to successful design verification• Certification

The need for certification and critical pre-certification measurement as part of design verification

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rf design considerations for 802.15.4 hardware … · proper antenna orientation is required to...

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